It is important to take a normal pressure reading with new cartridges present in the filter or filters cleaned with a product like Powder Blue D.E. & Cartridge Filter Cleaner to obtain the best cleaning results.
After inserting the clean or new cartridges into the filter, the reading on the pressure gauge is your normal pressure reading. All swimming pool systems are unique and in turn have unique normal running pressures.
Once the pressure on the gauge reads 10 psi above that of the normal pressure recorded with the clean filters its time to clean the filters again.
It is good practice to clean the cartridge filters every 6 months or even every 3 months for best performance.